1. Field of the Invention
The present invention relates to a solid-state image sensor, and particularly relates to a two-dimensional solid-state image sensor being characterized in the reading of signals.
2. Description of the Prior Art
Generally, a solid-state image sensor comprises photo sensors and a scanning circuit on a semiconductor material such as silicon, and if suitable photo sensors are selected, images from the visible region to an infrared region can be provided. As compared with a conventional image tube, a solid-state image sensor is advantageous in that it is compact, light in weight and has high reliability and in particular, few portions need to be adjusted in manufacturing a camera using such an image sensor. Thus, such a solid-state image sensor has attracted attention in various fields recently.
As a scanning circuit of a solid-state image sensor, a MOS switch system or a CCD (charge coupled device) system is conventionally used in most cases. In the former MOS switch system, spike noise due to the MOS switches used for reading the signals is mixed into the signals, which decreases the signal-to-noise ratio. In addition, the spike noise differs between the columns for reading the signals, which generates a so called fixed pattern noise to further decrease the signal-to-noise ratio, and as a result, a MOS switch system cannot be used for the detection of an extremely low light level application which requires a high signal-to-noise ratio. On the other hand, in the latter CCD system, particularly in a CCD interline system which has been widely used recently because it is capable of freely selecting photo sensors as in a MOS system, CCDs are disposed between the respective columns of photo sensors, and in order to enlarge the effective area of the photo sensors, it is desired tc minimize the area of the CCD portion in making a design. In addition, the charge transferring capacity of the CCDs is proportional to the storage gate area for one stage of CCDs if the CCDs have the same structure. Accordingly, if the area of the CCD portion is decreased, the maximum value of the charge to be handled become limited. Such a problem becomes serious particularly at the time of detecting a low level signal in a high level background as in an infrared ray solid-state image sensor. Therefore, a solid-state image sensor making less noise and having a large amount of charge to be handled has been desired.